A technique typically employed to determine the presence of hydrocarbons in earth formations is to electrically log boreholes drilled in these formations. Normally, formations saturated mostly with hydrocarbons will exhibit a high electrical resistivity, while formations saturated mostly with water or brine will exhibit a low electrical resistivity. However, it has been determined that the presence of shale in a formation substantially decreases the resistivity of the formation to the extent that commercially producing reservoirs in shaly formations display a resistivity that would otherwise indicate nonproductivity.
U. S. Pat. No. 3,895,289, issued July 15, 1975, discloses a relation between dielectric constant of a shaly sand formation, 100% water saturated, and a conductivity parameter related to shaliness, and provides a method of determining if there is enough shale in a formation that it must be taken into account when evaluating electrical logging results; but does not disclose a method for quantitatively determining the partial hydrocarbon saturation and partial water (brine) saturation of the formation.